The paper chart and magnetic compass lie at the very foundation of navigation. Ever since the first explorers began to navigate the seas better navigational practices, equipment and charts have also been explored and discovered. Currently, N.O.A.A. (National Oceanic and Atmospheric Administration) and others publish a variety of charts to facilitate safe navigation, including one of the best and most widely known types of charts used for navigation today called the “Mercator projection.”
Despite the automated abilities present in electronic navigational chart systems, certain limitations still exist. Furthermore, inherent in the very nature of electronic systems is the reliance upon external power. Prudent navigators still maintain their proficiency by planning, preparing, and tracking their voyages upon paper charts.
Either as a substitute for and an augmentation to electronic aids to navigation, paper charts remain at the bedrock of navigation. Paper charts are segments of a Mercator projection of the global surface of the earth onto a cylindrical map that is unrolled as flat on the navigation table. First presented by the Belgian, Flemish geographer and cartographer Gerardus Mercator, in 1569, the Mercator projection became the standard map projection for nautical purposes because of its ability to represent lines of constant course, known as rhumb lines or loxodromes, as straight segments.
Positions of places shown on a chart can be measured from the latitude and longitude scales on the borders of a standard chart. The advantage of Mercator projections is that it is a conformal projection, which is to say that the projection preserves angles locally. Thus, because the bearing angles are preserved, such charts facilitate what is known as dead reckoning navigation. Dead reckoning (DR) is the process of estimating one's current position based upon a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time, and course. Dead reckoning begins with a known position, or fix, which is then advanced, mathematically or directly on the chart, by means of recorded heading, speed, and time.
A bearing is the angle between the line joining the two points of interest and the line from one of the points to the north, such as a ship's course or a compass reading to a landmark. The basis of the Mercator projection is to preserve bearings. The Mercator projection, distorts distance but does so in a predictable manner. Specifically, degrees of latitude and the distances on the surface of the earth those degrees represent remain relatively constant across a chart when the spread of latitude is small. However, an unavoidable east-west stretching of the chart occurs, such that degrees of longitude become progressively short as the as distance away from the equator increases. Likewise, to preserve the angles, the north south distances are also stretched by a value also corresponding to distance from the equator. As a result, courses and bearings can be laid down on the chart to correspond to the actual angles of movement or sighting. Nonetheless, this preservation of angles comes at the cost of a predictable and calculable distortion of scale.
On nautical charts, the top of the chart is always true north, rather than magnetic north, towards which a magnetic compass points. Most charts also include a compass rose depicting the variation between magnetic and true north. They must also include cartographic scales to allow for the estimation of distance reflecting displacement on the chart. (Cartographic scale refers to the depicted size of a feature on a chart relative to its actual size in the world.) Because scale in a Mercator projection necessarily varies based upon distance from the equator and also the rate of variance also changes from place to place. At latitudes greater than 70° north or south, the Mercator projection is practically unusable. Yet, nearly all navigation occurs between 70° north and 70° south (Tromso, Norway being at 69° 40′N and Tierra del Fuego resting at 55°, 01′ S and there being no cities to the south within the Antarctic Circle), and thus the projection is extremely useful for marine navigation. In chartable areas, however, scale is necessarily provided in a legend on each chart.
Navigation charts have the advantage of portraying a proportionately small portion of the surface of the globe as opposed to the full Mercator projection. To that end, each chart can be scaled in accord with the use of the chart rather than to correspond to the scale of a full Mercator projection from 70° north to 70° south. Thus, a conventional chart will generally be drawn to have a standardized scale for the segment of the projection that is the subject of the chart. The scales of nautical charts range from 1:2,500 to about :5,000,000. Graphic scales are generally shown on charts with scales of 1:80,000 or larger, and numerical scales are given on smaller scale charts. NOS charts are classified according to scale as follows:
Sailing charts, scales are generally at 1:600,000 and smaller, and are for use in fixing the mariner's position approaching the coast from the open ocean, or for sailing between distant coastwise ports. On such charts the shoreline and topography are generalized and only offshore soundings, principal lights, outer buoys, and landmarks visible at considerable distances are shown.
General charts, scales are generally 1:150,000 to 1:600,000, and are used for coastwise navigation outside of outlying reefs and shoals.
Coast charts, scales are drawn in the range of 1:50,000 to 1:150,000, and are for inshore navigation leading to bays and harbors of considerable width and for navigating large inland waterways.
Harbor charts are drawn to scales larger than 1:50,000, for harbors, anchorage areas, and the smaller waterways.
Special charts, various scales, cover the Intracoastal waterways and miscellaneous small-craft areas.
One such standardized scale on the North South axis of the projection is 1:40,000. The East to West axis still varies as a function of the displacement from the Equator.
Because the key to navigation on a Mercator projection is the preservation of angles on the chart and their correspondence to angles on the earth's surface, orienting a straight edged rule in accord with an angle is an invaluable means of making fixes and laying down courses on a chart. To accomplish this, a number of instruments known as plotters have been invented and sold to aid mariners in keeping a straightedge oriented against a chart.
The Weems Parallel Plotter has been widely used for many years and is still relied upon today to satisfy many plotting navigational needs. Made of clear plastic it has various scales, a straight-edge and in some models a protractor. To maintain the orientation of the straightedge, the plotter includes rollers that allow translational movement of the Weems Plotter, and the straightedge it includes, over a paper chart while maintaining its orientation relative to the chart. In such a fashion, the Weems Plotter may be oriented on a compass rose and then by translational movement rolled to a current fix to establish a next leg in a course. The Weems Plotter is similarly used to mark bearings and with the bearings, to establish a fix. Despite its advantages, the WEEMS plotter also has some significant drawbacks and limitations for modern day use. To use the plotter appropriately, a very stable, expansive and platform is necessary to keep its rollers in full registration with the paper chart.
An improvement upon the WEEMS design is disclosed in U.S. Pat. No. 4,190,960, issued Mar. 4, 1980 to Warner. The improved plotter has a circular rotatable disk mounted on the body of the plotter with 360 degrees marked thereon. The disk must be rotated to determine the compass course of the flight path. The disk, then, can be used to maintain an angle relative to true north. Like the WEEMS, however, the Warner plotter is not suitable for use on a paper chart in a small craft.
Yet another plotter that attempts to improve on both these designs is U.S. Pat. No. 6,658,746 to Ganivet (Dec. 9, 2003). It's called a “PLOT'TIMER” and is designed for use with a marine or aeronautical chart; a one piece, thin, light weight, plastic, navigation plotter with at least one straight-edge and no moving parts. Used to determine true course, speed, and fuel requirements for a trip, it too calls for choosing various distance scales (miles, kilometers and statute & nautical miles). It has a reference point at the mid-point of the straight-edge (a small notch in the leading edge) and a boarder with angular markings representing the degrees of a compass rose. This plotter addresses fuel consumption and distance determination techniques. It is not a navigational tool at all.
All these plotters fail as a reliable, quick, and easy navigational tool. Each of these plotters can work well in optimal conditions, but all require; a level surface, steady, detailed manipulations and their readings can oft times be misread causing significant errors. In cramped quarters and unstable environments the use of rollers or sliding or rotating any of these plotters upon a chart table proves difficult and without a level surface it's hard to get even estimated bearings, distances and positions. In certain sea conditions these plotters are in and of themselves a navigational hazard underway; dangerous for overall use, especially in case of an EMERGENCY.
To ensure safe navigation what is needed is a chart plotter that is “Always Ready” to assist the navigator; especially in difficult conditions and in case of EMERGENCIES. What is lacking in the art is a ready easy means of applying a plotter that can be oriented to the chart without physical registration of rollers, thereby allowing rapid use in situations where sea conditions may not otherwise allow for the use of more primitive plotters.